This invention relates to a process of removing pollutants, consisting substantially of sulfur dioxides, from exhaust gases at temperatures below 150.degree. C. by means of a sorbent which contains calcium carbonate, calcium oxide and/or calcium hydroxide and which is included in a circulating fluidized bed system consisting of a fluidized bed reactor, a separator and a return line.
The combustion of fossil fuel results in the formation of flue gases, which, in dependence on the sulfur content of the starting material, may contain considerable amounts of sulfur oxides, particularly of sulfur dioxide. The exhaust gases produced in the continuously increasing number of refuse incinerating plants also contain sulfur oxides and, as a result of the incineration of plastics materials, which are always present in practice, contain hydrogen chloride and hydrogen fluoride as additional impurities. The legal provisions for the protection of the environment require such impurities to be removed from the gases before they are discharged into the atmosphere.
In by far the largest number of processes of purifying exhaust gases, the above-mentioned impurities are removed by wet-scrubbing, particularly with solutions or slurries of substances which will react with the pollutants (Ullmann's Encyclopadie der Techn. Chemie, 3rd edition, Volume 2/2 (1968), page 419).
Other processes effect a so-called dry purification of gas. That group also includes processes in which the sorbent is fed in a liquid phase or in which sorption is effected with an addition of water, provided that the liquid is transformed to the gas phase in the gas-purifying equipment. Steam may also be supplied instead of water. In said processes the gases are passed through a stationary bed of solids, such as activated carbon or brown coal coke, which react with the impurities. Exhaust gases can also be purified with the aid of a so-called moving bed of solids, which during their descent in the reactor are progressively laden and are finally discharged. Fresh solids at a rate corresponding to the discharge rate are supplied to the upper portion of the reactor (Meier zu Kocker "Beurteilung and Aussichten von Verfahren zur Rauchgasentschwefelung"; V.G.B. Kraftwerkstechnik 53 (1973), pages 516 et. seq.).
In another known process, e.g., sulfur oxides are removed from gases in that adsorbents are pneumatically charged into the gases to be purified, the resulting gas-solids dispersion is pneumatically moved through a reaction zone and subsequently through a holding zone, and the solids are subsequently separated from the gas. After certain regenerating steps have been performed, a partial stream of solids is finally recycled to the gas-purifying equipment (U.S. Pat. No. 3,485,014).
It is also known to remove pollutants from flue gases in a dry process in equipment which is connected to the flue of the fuel-burning furnace and comprises a boiler operated at a flue gas temperature of 700.degree. to 900.degree. C. That equipment comprises a fluidized bed, which completely occupies the cross-section of the flue gas duct, and/or a circulating fluidized bed, which is fed with an absorbent consisting, e.g., of calcium carbonate and/or magnesium carbonate (Laid-open German Application No. 30 09 366). The gas supply bottom of the fluidized bed is suitably cooled.
The essential disadvantages of the wet-scrubbing processes reside in that the sludge, which contains the sulfites and sulfates that have become available and may also contain chlorides and fluorides, can be disposed of only with difficulties and that the purified exhaust gases must be reheated. The known dry purifying processes using a fixed bed or moving bed have the disadvantage that the absorbents are coarse-grained so that their ability to combine with the impurities contained in the exhaust gas is utilized only to a low degree and that the reactor must have considerable dimensions because the permissible gas velocity is relatively low and gases at relatively high rates must be purified.
The essential disadvantages of the process disclosed in U.S. Pat. No. 3,485,014 reside in the requirement to divide the gas stream that is to be purified and to precisely meter the absorbent in a suitable apparatus, which must be wear-resistant. Another disadvantage resides in that the residence time of the gases in the reaction zone is not long enough for an adequate removal of the impurities or a considerable overall height is required.
Difficulties involved in the removal of pollutants from flue gases from 700.degree. to 900.degree. C. in accordance with Laid-Open German Application No. 30 09 366 reside in that the furnace must be controlled to provide flue gases at a temperature in the range from 700.degree. to 900.degree. C. whereas the flue gases are usually hotter as they leave the combustion chamber and are usually cooler as they leave the waste-heat boiler.
A disadvantage which is common to all dry purifying processes resides in that it is virtually impossible to use the sorbent to the same degree as in wet-scrubbing processes. Besides, in most cases the laden sorbent consists of a mixture of calcium sulfite and calcium sulfate and can be utilized only with difficulty and must usually be disposed of in a special dump.
In connection with the removal of sulfur oxides from flue gases in spray driers by means of lime-containing suspensions (Laid-open German Application No. 31 35 200) or in connection with the desulfurization of flue gases with solutions of suspensions of basic adsorbents, which are transformed to dry sulfite and sulfate (European Publication No. 74,772), it is known to subject the laden sorbents to an oxidizing treatment resulting in a formation of calcium sulfate. Whereas the problem of a disposal of wastes can be avoided to a large extent by said processes, they do not permit a utilization of the sorbent to a higher degree.